Rubber compositions for sidewalls of vehicle tires are required to provide excellent fuel economy. Known methods for improving the fuel economy include: the use of a butadiene rubber modified by a modifier; the use of a neodymium butadiene rubber synthesized in the presence of a neodymium catalyst and having a cis content of 95% by mass or more and a vinyl content of 1% by mass or less; and the use of a hybrid crosslinking agent. Yet, even a further improvement in fuel economy has been required these days.
A decrease in the amount of reinforcing filler (filler) such as silica and carbon black results in a lower tan δ and further improved fuel economy. This, however, also results in a lower complex modulus E*. Consequently, the handling stability, tensile strength at break, and elongation at break tend to decrease.
A known method for increasing the E* value is to use microfibers such as polyethylene terephthalate, fine paper fibers, and aramid fibers. These microfibers, however, are not bonded with the rubber matrix, and thus may become a starting point of cutting or breaking, which poses a problem in terms of flex crack growth resistance. The use of a high cis butadiene rubber having a high viscosity can also increase the E* value. This, however, decreases the dispersibility of filler, which tends to deteriorate the fuel economy. Yet, the reduced amount of filler results in a lower kneading torque, which tends to make it more difficult to disperse the filler in a favorable manner.
Additionally, the reduced amount of filler not only decreases the E* value but also tends to deteriorate the processability (extrusion processability) because, for example, the rubber composition may stick to a discharge opening of an extruder during extrusion of the rubber composition, or an extrudate of the rubber composition may have a non-uniform edge profile.
Hence, a method for improving the fuel economy, handling stability, tensile strength at break, elongation at break, and processability in a balanced manner is desired.
Patent Literature 1 suggests a method of using a 1,2-syndiotactic polybutadiene crystal-containing butadiene rubber, natural rubber and the like to improve the properties such as fuel economy and flex crack growth resistance. However, a technique to satisfy the fuel economy, handling stability, tensile strength at break, elongation at break, and processability at the same time is yet to exist.